Voltage converters such as DC-DC converters come in two forms. When an input voltage is to be stepped up in voltage by the converter, a boost or step-up configuration is provided. When the input voltage needs to be stepped down to a lower voltage by the converter, a buck converter is provided. Thus, a buck converter is a step-down DC-DC converter. Its design is similar to the step-up boost converter, and like the boost converter, it can be implemented as a switched-mode power supply that employs switches (e.g., a transistor and a diode), an inductor and a capacitor. Switched-mode power supplies can be efficient (95% or higher for integrated circuits), making them useful for tasks such as converting the main voltage in a telecom system (e.g., about −48 V) down to voltages needed by components such as a processor.
Most switched-mode power supplies require their power source's output impedance to be less than their respective input impedance (to frequencies above their loop crossover) for loop stability. There are classes of applications that are not able to meet this criterion due to system level considerations however. An example of this is a converter that must be hot-pluggable, which limits the permissible input capacitance to protect the input connector of the converter. Rather large, and potentially damaging, voltage and current oscillations between the converter switching elements and the input filtering can result from failing the stability criteria.